1. Field of the Invention
The present invention relates to a compound semiconductor light emitting device and a method of preparing the same, and more particularly, it relates to a GaN compound semiconductor light emitting device employing a substrate of GaAs, GaP, InAs or InP and a method of preparing the same.
2. Description of the Background Art
FIG. 5 is a sectional view showing the structure of a blue and green light emitting device (LED) employing a sapphire substrate, described in Nikkei Science, October 1994, P. 44, for example, which is now on the market.
Referring to FIG. 5, a clad or cladding layer 14, a light emitting layer 15, a clad or cladding layer 16 and a GaN epitaxial layer 17 are successively formed on an epitaxial wafer which is formed by a sapphire substrate 11, a gallium nitride (GaN) buffer layer 12 formed on the substrate 11, and a hexagonal GaN epitaxial layer 13 formed on the GaN buffer layer 12 in this blue and green light emitting device, while ohmic electrodes 18 and 19 are formed on the GaN epitaxial layers 13 and 17 respectively. In this blue and green light emitting device, the GaN buffer layer 12 is adapted to relax distortion resulting from the difference between lattice constants of the sapphire substrate 1 and the GaN epitaxial layer 13.
Referring to FIG. 5, this blue and green light emitting device employs insulating sapphire as the material for the substrate 11, and hence two types of electrodes must be formed on the same surface side while preparing the device. Thus, patterning by photolithography must be performed at least twice and the nitride layer must be etched by reactive ion etching, which thus requires complicated steps. Further, the substrate is hard to treat in element isolation, due to the hardness of sapphire. Regarding possible applications of the light emitting device, further, the sapphire substrate is disadvantageously inapplicable to a laser diode having an optical resonator defined by a cleavage plane, since sapphire is uncleavable.
In a conventional growth method, on the other hand, the growth temperature is so high that growth of a high In composition ratio cannot be attained in an InGaN layer which is an active layer, and hence a blue-green light emitting device is hard to prepare. Further, it is inevitably necessary to introduce zinc (Zn) as an emission center which leads to technical problems in application as a device since the emission wavelength is broad and the performance in a full color display is deteriorated.
To this end, an attempt has been made to employ conductive GaAs as the material for the substrate in place of sapphire having the aforementioned disadvantages. When the substrate is prepared from GaAs, however, an epitaxial wafer which rivals that employing the sapphire substrate cannot be obtained under conditions similar to those for the case of employing the sapphire substrate.
In relation to preparation of an epitaxial wafer with a GaAs substrate, therefore, various studies have been made in the art.
Among these studies, Nihon Kessho Seicho Gakkai-Shi Vol. 21, No. 5 (1994), Supplement S409 to S414 (hereinafter referred to as "literature 1"), for example, discloses an epitaxial wafer shown in FIG. 6.
Referring to FIG. 6, this epitaxial wafer comprises a GaAs substrate 21, a GaAs buffer layer 22 which is formed on the substrate 21, a GaN coating 23 which is obtained by nitriding a surface of the GaAs buffer layer 22 thereby replacing arsenic (As) with nitrogen (N), and a GaN epitaxial layer 24 which is formed on the GaN coating 23.
In preparing of this epitaxial wafer, the GaN epitaxial layer 24 is formed by OMVPE (organic metal vapor phase epitaxy). This OMVPE method is adapted to grow a GaN epitaxial layer on a substrate in a vapor phase by introducing a first gas including trimethylgallium (TMGa) and a second gas including ammonia (NH.sub.3) into a reaction chamber while heating only the substrate in the reaction chamber by high-frequency heating.
On the other hand, Jpn. J. Appl. Phys. Vol. 33 (1994) pp. 174-1752 (hereinafter referred to as "literature 2"), for example, discloses an epitaxial wafer shown in FIG. 7.
Referring to FIG. 7, a cubic GaN epitaxial layer 33 is formed on a substrate 31 which is previously provided on its surface with a cubic GaN buffer layer 32 by GS-MBE (gas source molecular beam epitaxy).
In preparing of the epitaxial wafer, the GaN epitaxial layer 33 is formed by hydride VPE (vapor phase epitaxy). This hydride VPE involves setting a substrate and a source boat containing Ga metal in a reaction chamber and introducing a first gas including hydrogen chloride (HCl) and a second gas including ammonia (NH.sub.3) into the reaction chamber while heating the overall reaction chamber with a resistance heater from the exterior, thereby growing a GaN epitaxial layer on the substrate in a vapor phase.
In the epitaxial wafer disclosed in the above cited literature 1, however, the GaN epitaxial layer is grown by OMVPE, as described above. When the GaN epitaxial layer is grown on the GaAs substrate by OMVPE, the film growth rate is extremely reduced as compared with the case of growing the layer on a sapphire substrate. In more concrete terms, the film forming rate for forming a film on a GaAs substrate is reduced to about 0.15 .mu.m/h., although a film forming rate of about 3 .mu.m/h. is attained in the case of forming a film on a sapphire substrate under the same conditions. Therefore, since a GaN epitaxial layer of about 4 .mu.m in thickness must be formed in order to apply the epitaxial wafer to a light emitting device, for example, almost one day is required for preparation in this method. Thus, preparing the layer by an epitaxial wafer by this method is unsuitable for industrialization, due to incapability of cost reduction.
According to this method, further, the treatment temperature cannot be much increased for growing the GaN epitaxial layer. Therefore, improvement of the characteristics of the obtained GaN epitaxial layer is limited.
In OMVPE, further, the ammonia is not sufficiently decomposed due to the so-called cold wall method of heating only the substrate in the reaction chamber. Thus, it is necessary to increase the supply quantity of ammonia in order to compensate for the incomplete decomposition then leading to a high V/III ratio (the ratio of a group V raw material to a group III raw material) of raw material supply.
According to Appl. Phys. Lett. 64 (1994), p. 1687, for example, it is calculated from the values of quantities of raw material introduction in growth that the V/III ratio in. GaN growth on a sapphire substrate is up to 6.0.times.10.sup.3 and that it is up to 1.1.times.10.sup.4 in GaInN gro with. Thus, OMVPE leads to enormous raw material consumption, and hence no epitaxial wafer can be prepared at a low cost.
According to OMVPE, further, the epitaxy is carried out at a high temperature of at least 800.degree. C., in order to facilitate decomposition of ammonia which is employed as the group V raw material. When the growth rate is thus increased, however a GaInN layer of high In composition is difficult to form, as described above.
In the epitaxial wafer disclosed the above cited literature 2, on the other hand, a substrate which is previously provided on its surface with a GaN buffer by GS-MBE must be prepared, in order to form a GaN epitaxial layer. In this case, the growth rate is so slow that formation of the GaN buffer layer on the GaAs substrate by GS-MBE is unsuitable for industrialization.
In employing hydride VPE, further, hetero growth requiring a plurality of sources as well as growth of a number of layers are so difficult that this method is unsuitable for practical utilization. In addition, two reaction chambers are required for preparing an epitaxial wafer by this method, since a buffer layer and an epitaxial layer are grown by different processes, and hence surface contamination caused by interruption of growth may disadvantageously come into question.
In the above cited literature 2, further, preparation conditions etc. for obtaining a GaN epitaxial layer of high quality/characteristics have not been studied in particular.